The present invention relates to an improved apparatus and method for removing particulates from gas streams, the apparatus being autoselectively regenerating (self-cleaning) in use.
Internal combustion engines and static hydrocarbon burning equipment tend to emit, via their exhaust systems, carbonaceous particles commonly referred to as particulates. Whilst unrelenting efforts are being expended towards reducing particulate emissions at source, particulate filters (traps) in the exhaust systems of such equipment are becoming essential to meet increasingly strict environmental legislation and public expectations.
Particulate filters which may be regenerated are known. In some cases these require regular removal from the equipment to which they are fitted followed by burning (oxidation) of the trapped particulates and refitment to the equipment. Particulate filters may be regenerated in this way by removal at the end of a working day, heating to a high temperature overnight to burn the collected particulates and refitting in the morning.
The removal, cleaning and refitting process has the disadvantage that the equipment is taken out of service for several hours, labour is required to remove, clean and refit the filter and the filter is generally subjected to a set cleaning process irrespective of the level of particulate build-up within it. Further, as the filter builds up its particulate content in use, resistance to exhaust gas flow is increased and thus an oversized filter may be required.
Taking mobile on-highway and off-highway vehicles out of service to regenerate the filter is particularly undesirable, and regeneration in situ offers clear advantages.
A conventional option is to provide two parallel filters, each one of which is alternately by-passed for regeneration in situ. However, there is a significant size and cost penalty with this system, the size of a particulate filter being typically the same as the swept volume of an associated naturally-aspirated engine and up to three times the swept volume of an associated turbocharged engine.
It is clearly desirable for regeneration to occur during use. However, for rapid oxidation of trapped particulate there must be sufficient free oxygen available, preferably included within the exhaust stream at its source. The free oxygen within the exhaust stream from a diesel engine ranges from 3% to 20%. In addition, unless pre-treated by, for example, catalyst or fuel additive means, the particulate must be at a temperature of at least 550xc2x0 C. for rapid oxidation to take place. However, this level of exhaust gas temperature occurs only for the upper part of an engine load-speed map, therefore extra energy must be put into the exhaust gas stream, the filter or the trapped particulate itself for it to be raised above 550xc2x0 C. for all engine operating conditions.
A conventional form of regenerating particulate filter incorporates a fuel burner system. This relies rely upon large amounts of heat being introduced into the exhaust system by fuelling, thus low particulate loadings can be oxidised. However, there is an unreliability concern with fuel burner systems associated with the required burner, ignition means, air pumps and isolation valves and the cost of such systems is relatively high. Further, most conventional systems require a method of measuring the amount of trapped particulate before regeneration commences, this being difficult and usually too inaccurate for optimum reliability of operation. Also, such a system will inherently have a detrimental effect on fuel consumption.
Other systems have been suggested which rely upon a control of the engine throttling. In essence, such systems have a throttle arrangement that is used intermittently to cause the engine to run richer at certain times to promote higher exhaust temperatures. Such systems have proven to be noisy in practice and also to have an adverse effect on both vehicle driveability and fuel economy.
Systems which use electrical resistance heaters to impart the extra energy have been proposed. However conventional electrical resistance heaters are likely to require significant energy input which can place an unacceptably high load on the engine""s battery/alternator system.
There have been suggestions that the use of microwave energy to impart the extra energy required might overcome some of these difficulties. However, such systems may have safety implications, and are not necessarily cheap.
Furthermore, reliability problems may arise with many conventional regenerating particulate filter systems as a result of their reliance on the exothermic nature of the oxidation to sustain the regeneration reaction. This leads to a basic requirement to have an optimum amount of trapped particulate in the filter to promote efficient oxidation. If there is too much particulate, then the heat released during oxidation may cause melting of the filter element or monolith leading to an almost total loss of filtration. If there is too little trapped particulate, there may be insufficient chemical energy to maintain oxidation and the process may die out leaving an unregenerated filter. Fuel burner systems depend less on the chemical energy in the particulate to sustain oxidation, but more on putting large amounts of heat energy into the exhaust stream. However, although this allows potentially lower particulate loadings to be oxidised, the unreliability of the fuel burner systems associated with the burner, ignition means, air pumps and isolation valves has already been outlined.
A known apparatus directed to avoid this operates by introducing fuel additives in controlled doses to reduce the temperature required to burn off particulates in conjunction with engine control strategies such as pilot injection via a common rail injection system to elevate the exhaust temperature, plus oxidation and reduced-temperature catalysts and a sensor for detecting when regeneration is needed. This system thus relies on an amalgamation of potentially complex and expensive equipment.
It will be seen from the above commentary that it is desirable for a particulate filter to be self-regenerating in use, under any load, in order to maintain filtering and gas-flow efficiencies above a certain level whilst keeping filter sizing to a minimum. It is also desirable that the filter is self-controlled to regenerate only when a predetermined level of particulates is present and to do so without requiring any external sensing means. It is further desirable that the regeneration process is economic in the use of any externally supplied energy or material, that the construction of the filter is also economic, and that the system is effective irrespective of types and compositions of fuel and engine operating conditions.
WO 94/07008 discloses an apparatus and a method said to provide a self-controlled, self-regenerating, particulate filter apparatus in which electrical spark and/or short time duration (xe2x80x9cpreferably between 0,001 sec and 0,1 secxe2x80x9d) are discharges oxidise and burn trapped particulates. It is asserted within ""07008 that the electrical discharges will occur only when carbon has accumulated to a sufficient thickness and homogeneity to become electrically conducting, which leads to a spark and/or arc discharge between the conducting layer and the electrodes when a certain limiting layer is reached.
In order to function, the apparatus would require at least a heavily loaded filter and it would not be expected to work at all at low loadings. Further, the preferred frequency of 50 Hz is within the human aural range and in a range which is not easily muffled, therefore audible noise emissions may result.
WO 94/07008 also recites that it is particularly suitable to divide the gas stream into two different streams, with a different filter being installed in each, and that it is particularly advantageous to provide the addition of air and/or oxygen to the gas stream during regeneration. It is clear that each of these operations will incur the need for additional apparatus which will add to the cost and size of the overall system.
WO 94/07008 describes simple filter tube or plate arrangements rather than conventional particulate filter monoliths. These simple arrangements present only a limited filter surface area for given size and, would need to be impracticably large in order to present a filtering surface of sufficient area to avoid unacceptable back-pressure. In contrast, a conventional particulate filter monolith may typically comprise a cylinder of 250 mm length and 150 mm diameter enclosing 2800 longitudinal cells, the walls of 1400 of which provide a very large filter surface area. It would not be possible to incorporate such a conventional monolith into the regenerating apparatus of ""07008.
It is an object of the present invention to provide a self regenerating particulate filter apparatus and method which mitigates the above disadvantages, to provide a self-regenerating particulate filter apparatus which is relatively practicable, economical and compact to manufacture and install.
It is a particular object of the present invention to provide a self-regenerating particulate filter apparatus which is able to autoselectively sense and oxidise trapped particulates even at low levels of particulate loading of the filter.
It is a further object to provide a self-regenerating particulate filter apparatus which will operate without any special apparatus for modifying the temperature or oxygen content of the exhaust gases or the oxidation temperature or other characteristics of the particulates.
It is a further object to provide a self-regenerating particulate filter apparatus which requires no exhaust back-pressure or temperature sensors nor any external control or timing devices.
It is a further object to provide a method for the operation of an autoselectively self-regenerating particulate filter apparatus.
According to the invention, an apparatus for removing particulates from a gas stream such as an exhaust stream of an internal combustion engine comprises a filter through which gas may be caused to flow, at least one first electrode, for producing an atmospheric glow discharge located near to but spaced apart from the filter, and at least one second, counter electrode, the said first and second electrodes being connected to an AC voltage supply generating an AC voltage in a frequency within the range of 1 kHz to 200 kHz.
As has been noted, for efficient oxidation the particulate must be at a temperature of at least 550xc2x0 C. Since such levels of exhaust gas temperature are not encountered throughout all engine operating conditions, extra energy must be put into the exhaust gas stream, the filter or the trapped particulate itself for its effective temperature to be raised above 550xc2x0 C. The apparatus of the present invention provides the required extra energy by an electrical input of relatively low power consumption in the form of an atmospheric glow discharge.
The invention permits the automatic regeneration in use of a particulate filter, for example of a type including a ceramic monolith. Capacitive coupling combined with high electric field strength enables the deposited particulate to be autoselectively sensed by the system, and the particulate to be oxidised rapidly during a discharge almost the moment it arrives on the filter surface, so that the invention ensures efficient oxidation of particulate at relatively low levels of particulate loading within the filter. Thus there is no requirement to provide pairs of filters in parallel, each one of which is alternately bypassed for regeneration in situ, as required by some conventional systems, and the filter may be smaller than conventional since it will not be allowed to have a particulate build-up which could lead to high levels of exhaust back-pressure and/or damage to the filter.
The frequency of operation of the AC source is of particular importance in determining the efficiency of operation of the device. The present invention uses frequencies which are relatively low. Considerable advantages particularly in relation to cost, size, and ease of fitment to a vehicle can arise from the use of lower frequency sources, which can be relatively easily and cheaply generated with compact, reliable and robust components. At higher frequencies the cost and complexity are greater and the circuits generally less robust. These factors determine the upper limit of optimum frequency range. AC sources of 200 kHz or less are used, and sources of 50 kHz or less are preferred to maximise use of solid state technology.
By contrast, at lower frequencies capacitive coupling of the discharge to the filter, which decreases as the frequency decreases, is less effective. The invention takes advantage of the fact that significant capacitive coupling is produced at considerably lower frequencies than has been suggested in the prior art. Nevertheless, if frequencies are too low, the reactance of the capacitive path becomes relatively high, and in order to function the apparatus is likely to require to be heavily loaded, and would not be expected to work at low loadings due to the low capacitive couplings. The present invention is intended to provide stable and efficient regeneration of the filter at relatively low loadings, and therefore frequencies of operation below 1 kHz are not generally considered desirable, and frequencies above 10 kHz are likely to be preferred.
It is desirable that the frequency of operation of the AC source lies outside the human aural audible range. In consequence, a preferred frequency range for the voltage source is 18 kHZ to 30 kHz, more preferable 20 kHz to 25 kHz, with frequencies of around 20 kHz being shown to be particularly effective.
The voltage source preferably provides an open circuit output voltage of between 5 kV and 25 kV, with an open circuit voltage output of around 10 kV being particularly preferred.
Preferably, the or each first electrode is situated at a first end of the filter. The or each counter electrode is spaced away from the or each first electrode. The or each counter electrode is preferably situated in a location which lies downstream in use from the or each first electrode, and may be located at the side (e.g. radially disposed) or at a downstream end of the filter. The or each second electrodes is preferably situated adjacent to and may be in electrical contact with a second downstream end of the filter.
The electrodes, and in particular the first electrode(s), which generate the atmosphere glow discharge, may comprise point electrodes. The electrodes are preferably stabilised by resistors.
The invention covers embodiments in which a single first electrode and a single counter electrode are used, although in some applications it may be preferred that a plurality of counter electrodes and/or a plurality of first electrodes are used.
The or each first and/or second electrodes may comprise a plurality of discharge locations. Such a plurality of discharge locations may be provided by means of an array of point electrodes. Alternatively, one or more plate electrodes may be provided, each of which plate electrodes comprises a conducting plate any part of the surface of which may provide a location for the discharge. Alternatively, one or more network-like electrodes may be provided, in the form of a perforated sheet of conducting material or a wire mesh of conducting material. The conducting material in the above cases preferably comprises metallic material, such as for example copper. Combinations of these types of electrode may be applicable to particular applications.
An alternative form of electrode which may be practicable depending upon the filter material may be provided in the form of a layer of conducting material coated on or impregnatedly bonded to an end face of the filter. Such a surface electrode may be formed by dipping part or all or an end of the filter into a bath of suitable conductor, such as molten silicon, such that a small quantity is absorbed in to the end face to form a discrete and securely bonded surface electrode layer, or by spraying or otherwise coating part or all of the end of the filter with a conductor. Control of configuration and/or composition of the conductor provides a means of electrode stabilisation and control of performance.
The present invention is not specific to a particular type of particulate filter, and any filter design which generally removes particulates from a gas flow by trapping the particulates on a surface such as a ceramic surface may be utilised in accordance with the invention. For example, filters may comprise ceramic fibre, foam, membrane, sheet or pad devices. Nevertheless, it is a particular advantage of the present invention is that it allows the use of conventional monolithic particulate filters. Thus, in accordance with the invention, the filter preferably comprises at least one monolith comprising at least one tubular cell defined by a porous ceramic wall, and preferably a plurality of such cells each so defined and having alternate ends blocked off, so that the ceramic walls act as a filter surface in a manner which will be familiar from conventional monolith filters.
As has been indicated, the apparatus of the present invention will efficiently auto-selectively seek and oxidise deposited particulates within the monolith.
In consequence, because back pressure can be minimised a filter in accordance with the invention may be smaller that might hitherto have been the case, and in particular a monolith may be significantly shorter than that normally required by conventional filter apparatus. Conventional monoliths may function with a design loading of as high as 90% prior to regeneration, and hence with as little as 10% of clean surface area available. Therefore, although a lower limit to monolith size is still mandated by the need for sufficient porous wall surface area for the exhaust gases to pass through, a filter in accordance with the present invention will function efficiently where the filter comprises a monolith having a correspondingly reduced depth in comparison with conventional filters. For example, a depth in the range of 10 mm to 25 mm, in comparison with the sizes of the order of 100 mm or more encountered in conventional devices, may be appropriate.
To minimise problems of gas flow turbulence, the apparatus may further include a gas flow tube stack, comprising a plurality of tubular cells, placed in axial alignment with but spaced apart from and upstream of a first end of the monolith. A surface electrode may be provided by impregnatingly bonding or depositing a layer of conducting material on a portion of the tube stack.
The filter and associated apparatus may be mounted within a filter body, which body is provided with apertures for the ingress and egress of flowing gases such as exhaust gases. The filter body may itself form or integrally include the counter electrode, either radially or at a downstream end of the filter. Such an arrangement allows control over the direction of gas flow for optimum performance. For example, to avoid problems which may arise where discharges do not reliably penetrate fully through large monoliths, it may be desirable to position one or more monoliths within a filter body such that gas flow as it enters the body is in a direction lateral to the direction of gas flow through the monolith. In this way, the overall surface area presented by the monolith can be increased without increasing the through-depth of the monolith, thus obviating discharge penetration problems associated with excessively large monolith depth.
In accordance with a further aspect of the invention, a method of removing particulates from a gas stream comprises causing the gas to flow through a filter so that particulates are separated from the gas flow and trapped by the filter, positioning at least one first electrode near to but spaced apart from the filter, providing at least one counter electrode, and applying an AC voltage at a frequency in the range of 1 kHz to 200 kHz between the electrodes so as to generate an atmospheric glow discharge from the at least one first electrode.
In accordance with the method, the trapped filter particles are ignited and oxidised in situ, with the advantages outlined above in the description of the apparatus. A safe and effective low power source is used, and the method is applicable to conventional monolith-type filters. Further preferred features of the method will be readily understood by the skilled person by analogy with the features above described of the filter apparatus.